it shouldn't take longer than a 1/2 hour I suppose. they will weigh you in, you will get to see how you did on takeoff and then they will explain the plan to you and tell you which one you are on. then you can start drooling on all the food added back tomorrow
And don't be discouraged if you don't show a bunch of loss right off the bat today.. It takes a while for many people to kick start their system.. You are about to throw your body into shock by starting to eat better than you ever have.